Exhaust gas turbocharger with plain bearing for reducing fluid turbulence

ABSTRACT

An exhaust-gas turbocharger may include a rotor shaft arranged with at least one bearing-shaft portion in a radial bearing. The radial bearing may be a plain bearing configured to slide on a fluid film. The at least one bearing-shaft portion may be subdivided into two outer segments by an inner segment. The two outer segments may be arranged on the respective axial end region of the radial bearing and the inner segment may be positioned between the outer segments. The inner segment may be arranged in the area of an opening of a fluid channel. The turbocharger may include at least one transverse groove running inclined relative to the circumferential direction of the rotor shaft arranged in at least in one of the outer segments. The at least one transverse groove may include a groove depth having a continuously decreasing development in a fluid flow direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German Patent Application 10 2010 022 574.6, filed on Jun. 2, 2010, and International Patent Application PCT/EP2011/058526, filed on May 25, 2011, both of which are hereby incorporated in their entirety.

TECHNICAL FIELD

The present invention relates to an exhaust-gas turbocharger having a rotor shaft, with the features of the preamble of Claim 1. The invention furthermore relates to a method for operating an exhaust-gas turbocharger.

BACKGROUND

From EP 1 972 759 A2 an exhaust-gas turbocharger is known, which in a radial bearing, in which a shaft of the exhaust-gas turbocharger is mounted in a rotation-moveable manner, comprises a plurality of axially running grooves. Here, the grooves can have a V-shape or U-shaped in profile. The axially running grooves in this case advantageously make possible a further development of the rotor dynamics of such an exhaust-gas turbocharger, since an exhaust-gas turbocharger with such axially running grooves in the radial bearings has a higher resistance relative to a movement of the rotor shaft known as self-excited sub-synchronous vibration. This is achieved through a reduction of fluid turbulences in the fluid film arranged between the radial bearing and the rotor shaft, in particular in the low load range.

The designation fluid turbulence likewise includes the technical term “oil whirl”. The occurrence of such fluid turbulences is known with the use of plain bearings in charging devices and is caused because of the rotating rotor shaft about the bearing centre, wherein in the fluid film arranged between the radial bearing and the rotor shaft, inner and outer fluid turbulences can be distinguished. Fluid turbulences occur in particular also at high rotational speeds and cause, in particular at higher rotational speeds, a wobbling movement of the rotor or of the rotor shaft in cylindrical and conical vibration modes, which is also known as “self-excited sub-synchronous vibration”. In particular the inner fluid turbulence induces a constant tone because of the flow of the inner fluid film, the frequency of which, dependent on the respective fluid temperature, lies in an order of magnitude of the half-frequency of the rotor rotational speed. Here, the constant tone induced by the fluid turbulence can have a broad-band frequency of 600 to 900 Hz, which occurs at corresponding engine rotational speeds of 1,500 to 3,500 rpm. The self-excited vibration that occurs through the fluid turbulence, which can excite the rotor deflection into an unstable state, can result in a bearing damage in unfavourable operating conditions. Such a destructive vibration can occur when the frequency of the constant tone of the fluid turbulence is equal to the critical frequency of the first bending order of the rotor-bearing system.

In addition, a frequent start-stop operation with currently usual rotor-bearing systems in turbines or charging devices, can lead to an increased wear of the rotor-bearing system. When for example in a motor vehicle with a charging device the engine is briefly switched off and on again for example during a city drive and in front of traffic lights, the wear on the radial bearing or on the rotor shaft can increasingly occur because of a lack of fluid in the fluid film upon frequent use of the start-stop operation and in particular the starting operation.

SUMMARY

The present invention deals with the problem of stating an improved or other embodiment for an exhaust-gas turbocharger and for an associated operating method, which is characterized in particular by a reduced wear.

According to the invention, this problem is solved through the subjects of the independent claims. Advantageous embodiments are subject of the dependent claims.

The invention is based on the general idea of sub-dividing a rotor shaft of the exhaust-gas turbocharger, which is arranged with at least one bearing-shaft portion in a radial bearing designed in particular as plain bearing in a rotationally moveable manner sliding on a fluid film, into two outer segments arranged on the respective radial bearing end region and an inner segment positioned between the outer segments, wherein at least in one of the outer segments at least one transverse groove running inclined relative to the circumferential direction of the rotor shaft is arranged.

Surprisingly it was established that by arranging transverse grooves in a bearing-shaft portion of the rotor shaft running inclined relative to a circumferential direction of the rotor shaft, the wear of rotor shaft and radial bearing can be substantially reduced.

The rotor shaft carries for example a compressor wheel of a compressor of the exhaust-gas turbocharger and/or a turbine wheel of a turbine of the exhaust-gas turbocharger. Practically, the rotor shaft is rotatably mounted in a bearing housing of the exhaust-gas turbocharger for this purpose. Usually, the bearing housing is located axially between a compressor housing and a turbine housing.

Advantageously, a pumping effect, which is brought about through the movement of the rotor shaft, is established through the at least one transverse groove arranged inclined in at least one outer segment. Because of the rotation of the rotor shaft and through a certain position of the transverse groove, fluid is shovelled out of the fluid film of the inner segment into the transverse groove and transported along said transverse groove to the at least one outer segment. Because of this, fluid is fed to the fluid film arranged in the at least one outer segment. Thus one succeeds during a starting operation of the rotor shaft to establish the fluid film arranged in the outer segment faster because of the pumping effect through the inclined transverse groove, as a result of which a wear reduction during an in particular frequent start-stop operation is possible. Furthermore, the pressure in the fluid film is increased at least in the one outer segment as a function of the rotational speed of the rotor shaft. Through this increased pressure in the fluid film with running operation, the bearing stiffness is increased and the constant tone with respect to the inner fluid turbulence reduced. In addition, because of the increased bearing stiffness and the reduction of the constant tone, the radial bearing-rotor shaft system with such a transverse groove is less sensitive to vibrations, imbalance vibrations and deflections. Altogether, the wear during the entire operation of such a charging device or turbine with respect to the radial bearing-rotor shaft system is clearly reduced because of this.

A further core idea of the invention is a method for operating a rotor shaft, wherein a bearing-shaft portion of the rotor shaft is arranged rotationally moveably in a radial bearing in particular designed as plain bearing and sliding on a fluid film, and at least comprises one transverse groove running inclined relative to the circumferential direction of the rotor shaft, wherein through the pumping effect of the at least one transverse groove a fluid pressure is established in a fluid film arranged between the rotor shaft and the radial bearing.

A rotor shaft designed in this manner can be used together with at least one turbine wheel as rotor in a charging device or a turbine. Here, a bearing-shaft portion of the rotor shaft is arranged in a radial bearing and positioned in a sliding and rotationally moveable manner on a fluid film arranged between the radial bearing and the rotor shaft.

Here, the radial bearing can be designed as rotating or non-rotating floating bush bearing. Quite in general, the use of a slide bearing is preferred here, wherein instead of a floating bush bearing a fixed bush bearing can also be employed. The radial bearing comprises at least one fluid channel substantially running radially, which is preferably arranged in the middle of the radial bearing. Via this fluid channel, the fluid film arranged between the rotor shaft and the radial bearing can be supplied with fluid or fluid be transported away from the fluid film.

In the region of the bearing-shaft portion arranged in the radial bearing, the rotor shaft can be sub-divided into a plurality of segments. Here, an outer segment each is arranged in the radial bearing end region on the rotor shaft in the bearing-shaft portion and an inner segment between the two outer segments. Here, the inner segment is positioned in such a manner that it is substantially arranged in the region of the opening of the fluid channel of the radial bearing. Thus, fluid is fed to the bearing-shaft portion or discharged from the bearing-shaft portion via the inner segment.

In at least one outer segment, at least one transverse groove is positioned. Here, at least one transverse groove for each outer segment is advantageously arranged in both outer segments. Particularly preferably, 8, 14, 16 or 20 transverse grooves each are arranged in an outer segment.

The inner segment can be designed free of transverse grooves or at least one transverse groove arranged in an outer segment can extend into the inner segment. In both cases, free of transverse grooves or having at least one transverse groove, the inner segment can be provided with at least one annular groove running in circumferential direction. Preferably, the inner segment comprises only one annular groove, which in a most preferable manner extends in longitudinal direction of the rotor shaft over the entire inner segment.

Through such an annular groove, the fluid feed/discharge over the inner segment is facilitated. If in addition the at least one transverse groove in a connecting region of the outer segment to the inner segment is fluidically connected to the annular groove, the transport of the fluid to the inner segment or away from the inner segment is advantageously improved. In addition, the fluid arranged in the annular groove constitutes a certain fluid reservoir, which can be used in particular during the start-stop operation for the rapid establishment of the fluid film arranged between the rotor shaft and the radial bearing and positioned in the region of the outer segment.

The at least one transverse groove in any case is arranged running inclined relative to the circumferential direction of the rotor shaft. However, the orientation of the transverse groove should be selected as a function of the direction of rotation and of the axial flow direction of the fluid.

Circumferential direction is to mean the direction in which a circumference runs on the surface of the rotor shaft. The direction of rotation constitutes the direction in which the rotor shaft rotates. The longitudinal direction runs along the longitudinal axis of the rotor shaft. The fluid flow direction constitutes that direction in which the fluid flows in the region of the radial bearing. Here, two directions can be distinguished. Thus, fluid can be fed to the radial bearing via the fluid channel, so that a fluid flow direction from the inner segment to the outer segment is established. If the fluid is discharged from the radial bearing via the fluid channel, a flow direction from the at least one outer segment to the inner segment is established.

Upon a flow direction from the inner segment to the outer segment, the at least one transverse groove runs in the direction of rotation from the outer segment to the inner segment as a function of the direction of rotation and of the fluid flow direction.

Upon a fluid flow direction from the outer segment to the inner segment, the at least one transverse groove runs from the inner segment to the outer segment in the direction of rotation. In order to determine the respective orientation of the transverse groove here, one touches down on the inner segment or outer segment and moves on the rotor shaft in direction of rotation and inclined to the circumferential direction to the respective other segment. This then produces the orientation of the transverse groove on the rotor shaft.

Here, the at least one transverse groove is arranged under a predefined angle to a longitudinal direction of the rotor shaft imagined on a rotor shaft surface. This angle can take up a value of preferably 15-30°, particularly preferably of 30-45° and most preferably of 45-60°.

In the event that the transverse grooves protrude into the inner segment, an annular region can still remain in the inner segment which is free of transverse grooves. The transverse grooves however can also intersect in the inner segment. If transverse grooves are arranged in both outer segments, these transverse grooves can be arranged symmetrically in the outer segments, or offset relative to one another.

The cross section of such a transverse groove can be rectangular, square or triangular and/or half-round or ellipsoid in portions. Preferably, the edges orientated towards the rotor shaft surface are provided with chamfers. This chamfer can be designed as rounding or broken edge as well as a bevel. Because of the chamfers, the wear, in particular in the radial bearing, is clearly reduced.

Because of the orientation of such a transverse groove, the fluid is pressed out of the fluid film into the transverse groove through the rotary movement of the rotor shaft and transported along said transverse groove in flow direction. Through this pumping effect, the pressure in the fluid film can be increased in the region of the outer segments and the bearing stiffness improved because of this.

Advantageously, the pressure build-up in the fluid film can be improved through a falling groove depth in fluid flow direction along the transverse groove. Through the pumping effect, fluid is transported along the fluid flow direction within the transverse groove and because of the falling depth, the fluid is increasingly dammed up in the transverse groove along the fluid flow direction. Because of this, the pressure build-up in the fluid film is improved.

Further important features and advantages of the invention are obtained from the subclaims, from the drawings and from the associated Figure description by means of the drawings.

It is to be understood that the features mentioned above and still to be explained in the following cannot only be used in the respective combination stated but also in other combinations or by themselves, without leaving the scope of the present invention.

Preferred exemplary embodiments of the invention are shown in the drawing and are explained in more detail in the following description, wherein same reference characters refer to same or similar or functionally same components.

BRIEF DESCRIPTION OF THE DRAWINGS

It shows, in each case schematically:

FIG. 1 a bearing region of an exhaust-gas turbocharger,

FIG. 2 a bearing-shaft portion arranged in a radial bearing,

FIG. 3 a plurality of transverse grooves arranged in an outer segment,

FIG. 4 different embodiments of a cross section of the transverse groove,

FIG. 5 a falling groove depth along the fluid flow direction,

FIG. 6 a plurality of embodiments of a bearing-shaft portion.

DETAILED DESCRIPTION

A bearing region 1 of an exhaust-gas turbocharger which is not otherwise shown comprises at least one radial bearing 2, 2′, in which a rotor shaft 3 of a rotor 4 is rotationally moveably mounted. Usually, the bearing region 1 is surrounded by the bearing housing 5 of the exhaust-gas turbocharger.

The radial bearing 2, 2′ comprises at least one fluid channel 6, with which fluid can be fed to the bearing region 7 or with which fluid can be discharged from the bearing region 7. Between the rotor shaft 3 and the radial bearing 2, 2′, a fluid film 8 is arranged, on which the rotor shaft 3 can slide. The fluid channel 6, with other devices not shown in FIG. 1, among other things serves for the steady exchange of the fluid film 8. By way of a fluid supply device 9, 9′, the fluid channel 6 and the radial bearing 2, 2′ can be supplied with fluid. The fluid supply device 9, 9′ can be designed as crescent groove.

FIG. 2 shows a detail of the charging region 1 in the region of the radial bearing 2, 2′. Here, a bearing-shaft portion 10 of the rotor shaft 3 has a segment-like structure. In the region of the fluid channel 6, an inner segment 11 is thus arranged on the rotor shaft 3. In the respective bearing end region 12, 12′, an outer segment 13, 13′ each is arranged on the rotor shaft 3, so that the two outer segments 13, 13′ delimit the inner segment 11. Here, the diameter of the rotor shaft 3 is designated D_(W) and the diameter of the fluid channel 6 is designated D_(F).

In the outer segments 13, 13′, a plurality of transverse grooves 14 is positioned so that these run inclined relative to the circumferential direction. Here, the transverse grooves 14 run in direction of rotation 15 from the respective outer segment 13, 13′ to the inner segment 11.

Here, the transverse grooves 14 extend linearly in a radial projection. An embodiment, wherein the transverse grooves 14 extend curved in a radial projection is likewise conceivable.

FIG. 2 shows a symmetrical embodiment of the transverse grooves 14, wherein the transverse grooves 14 with respect to the bearing centre plane 16 are positioned mirror image-symmetrically on the rotor shaft 3 in the bearing-shaft portion 10. In this embodiment, the fluid flow direction 17 runs from the inner segment 11 to the outer segments 13, 13′. The length of the bearing-shaft portion 10 is designated L_(W).

FIG. 3 shows a detail of an outer segment 13, 13′. The fluid flow direction 17 runs again from the inner segment to the outer segment as drawn in and the direction of rotation 15 runs analogously to the FIG. 2. Furthermore, two section lines IV, V are drawn in, each of which are shown in the FIGS. 4 and 5.

FIGS. 4 a and 4 b show two different embodiments of a transverse groove 14. In FIG. 4 a, a rectangular shape is shown, while the FIG. 4 b has a half-round shape. In both cases, the edges 18 of the transverse grooves 14 are provided with chamfers 19. Here, the chamfer 19 can be embodied as broken edge or rounding.

The section V of FIG. 3 is shown in FIG. 5. It shows a transverse groove 14, which is designed in fluid flow direction 17′ with falling groove depth 20, 20′ within the transverse groove 14. Because of this, likewise in the region of the transverse groove 14, the spacing between the inner bearing region 7 and the transverse groove bottom 21 drops in fluid flow direction 17. Through this falling groove depth design one succeeds in establishing the fluid film 8 more quickly and higher.

The FIGS. 6 a to 6 d shows different embodiments of the bearing-shaft portion 10. In the FIG. 6 a, the fluid flow direction 17 runs from the inner segment 11 to the outer segments 13, 13′ and the direction of rotation 15 of the rotor shaft 3 as drawn in. With respect to the bearing centre plane 16, a symmetrical embodiment is shown here. However, a non-symmetrical design is likewise conceivable, wherein the transverse grooves 14 are arranged offset to one another in circumferential direction. With respect to an imaginary longitudinal direction 22 of the rotor shaft on the rotor shaft surface, the respective transverse groove is arranged inclined at an angle β. This angle β can preferably have a value between 15 and 30°, particularly preferably a value between 30 and 45° and most preferably a value between 45 and 60°.

The embodiment shown in the FIG. 6 b is constructed analogously to the embodiment of FIG. 6 a with the only difference being that the fluid flow direction 17 is orientated from the outer segment into the inner segment. Accordingly, the transverse grooves run as shown.

FIG. 6 c shows an embodiment of the bearing-shaft portion 10 with a groove-free inner segment 11. The width of the inner segment is designated B_(i), while the widths of the outer segments are designated B_(a), B_(a)′. In a particularly preferred embodiment, the widths B_(a), B_(a)′ of the outer segments are identical in size.

The embodiment of FIG. 6 d comprises an annular groove 23 extending over the entire inner segment region. Preferably, the transverse grooves in this case are designed in a connecting region 24, 24′ of the respective outer segment 13, 13′ to the inner segment 11 so that they are fluidically connected to the annular groove 23. Preferably, the lowest point of the transverse groove bottom 21 here is located at the height of the annular groove bottom 25. In a further embodiment, the transverse groove bottom 21 can also be arranged below the annular groove bottom 25 viewed from the rotor shaft surface. 

1. An exhaust-gas turbocharger comprising: a rotor shaft being arranged with at least one bearing-shaft portion in a radical bearing, the radial bearing being a plain bearing being rotationally moveable and configured to slide on a fluid film, wherein the at least one bearing-shaft portion is subdivided into two outer segments by an inner segment, the two outer segments arranged on the respective axial end region of the radial bearing and the inner segment positioned between the outer segments, further wherein the inner segment is arranged in the area of an opening of a fluid channel of the radial bearing, and at least one transverse groove running inclined relative to the circumferential direction of the rotor shaft arranged in at least in one of the outer segments, wherein the at least one transverse groove includes a groove depth including a continuously decreasing development in a fluid flow direction.
 2. The exhaust-gas turbocharger according to claim 1, wherein the at least one transverse groove runs along a direction of rotation of the rotor shaft from the respective outer segment to the inner segment and the fluid flow direction runs from the inner segment to the outer segments.
 3. The exhaust-gas turbocharger according to claim 1, wherein the at least one transverse groove is arranged at a predetermined angle β relative to a longitudinal direction of the rotor shaft.
 4. The exhaust-gas turbocharger according to claim 1, wherein the at least one transverse groove protrudes into the inner segment.
 5. The exhaust-gas turbocharger according to claim 1, wherein a spacing between a rotor shaft-rotation axis and a transverse groove-free inner segment-surface region is equal to a spacing between the rotor shaft-rotation axis and a transverse groove-free outer segment-surface region.
 6. The exhaust-gas turbocharger according to claim 1, wherein a spacing between a rotor shaft-rotation axis and a transverse groove-free inner segment-surface region is smaller than a spacing between a rotor shaft-rotation axis and a transverse groove-free outer segment-surface region.
 7. The exhaust-gas turbocharger according to claim 1, wherein a spacing between a rotor shaft-rotation axis and a transverse groove-free inner segment-surface region is equal to a spacing between a rotor shaft-rotation axis and a deepest point of the at least one transverse groove in a connecting region of the at least one transverse groove to the inner segment.
 8. The exhaust-gas turbocharger according to claim 1, wherein a groove depth of the at least one transverse groove decreases along a fluid flow direction within the at least one transverse groove.
 9. The exhaust-gas turbocharger according to claim 1, wherein the at least one transverse groove includes groove edges having at least one of rounding, a chamfer or a broken edge.
 10. The exhaust-gas turbocharger according to claim 1, wherein a cross section of the at least one transverse groove is at least one of rectangular, at least partially ellipsoid, and at least partially half-round.
 11. The exhaust-gas turbocharger according to claim 1, wherein the at least one transverse groove runs along a direction of rotation from the inner segment to the respective outer segment and a fluid flow direction runs from the outer segments to the inner segment.
 12. The exhaust-gas turbocharger according to claim 1, wherein the at least one transverse groove includes at least two transverse grooves, the two transverse grooves being interconnected in the inner segment.
 13. The exhaust-gas turbocharger according to claim 2, wherein the at least one such transverse groove is arranged at a predetermined angle β relative to a longitudinal direction of the rotor shaft.
 14. The exhaust-gas turbocharger according to claim 13, wherein the at least one transverse groove protrudes into the inner segment.
 15. The exhaust-gas turbocharger according to claim 14, wherein a spacing between a rotor shaft-rotation axis and a transverse groove-free inner segment-surface region is equal to a spacing between the rotor shaft-rotation axis and a transverse groove-free outer segment-surface region.
 16. The exhaust-gas turbocharger according to claim 14, wherein a spacing between a rotor shaft-rotation axis and a transverse groove-free inner segment-surface region is smaller than a spacing between a rotor shaft-rotation axis and a transverse groove-free outer segment-surface region.
 17. The exhaust-gas turbocharger according to claim 14, wherein a spacing between a rotor shaft-rotation axis and a transverse groove-free inner segment-surface region is equal to a spacing between a rotor shaft-rotation axis and a deepest point of the at least one transverse groove in a connecting region of the at least one transverse groove to the inner segment.
 18. The exhaust-gas turbocharger according to claim 17, wherein a groove depth of the at least one transverse groove decreases along a fluid flow direction within the at least one transverse groove.
 19. The exhaust-gas turbocharger according to claim 18, wherein the at least one transverse groove includes groove edges having at least one of a rounding, a chamfer or a broken edge.
 20. The exhaust-gas turbocharger according to claim 19, wherein a cross section of the at least one transverse groove is at least one of rectangular, at least partially ellipsoid, and at least partially half-round. 